1. Field of the Invention
The present invention relates to digital-to- analog (D/A) converters. More particularly, the present invention relates to methods and apparatus for digital calibration of D/A converters.
2. Description of the Related Art
The most widely used D/A converters are of switched current type (current-mode) because of their operating speed and size. A current-mode D/A converter includes an N binary scaled current sources (for example, current cells) and associated switches. FIG. 1 illustrates a conventional current mode D/A converter current cell. As shown, switch S.sub.1 connects current I to a common summing node N.sub.S for a corresponding data bit D.sub.i of a "1" to provide a current cell output current I.sub.i along a signal output line current.sub.-- out, while a second switch S.sub.2 steers the current I to a ground node GND for a data bit D.sub.i of a "0". In a differential D/A converter cell, switch S.sub.2 is connected to a complementary output node N.sub.C as shown in FIG. 2.
The total output current I.sub.OUT of the current mode D/A converter is then determined by a sum of each of the output currents I.sub.i of the N current cells in the D/A converter as shown by the following expression. ##EQU1## where N is the number of current cells in the current mode D/A converter architecture, and further, where each current cell corresponds to a current cell output current I.sub.i for a corresponding data bit D.sub.i.
As can be seen from equation (1), the total output current I.sub.out of the current mode D/A converter is an accurate analog equivalent of the binary input code D.sub.i, provided that the individual current cells I.sub.i are binary scaled and highly accurate.
For D/A converters with a large number of bits, the required accuracy can be obtained by trimming. The cost of the manufacturing for the trimming process and the associated circuit and/or process complexity (such as non-volatile memory) are primarily responsible for the development of self-calibrated converters. Self-calibration methods using dynamic analog calibration is discussed in Groeneveld, D. W. J, et al., "A self-calibration technique for monolithic high resolution D/A converters", IEEE Journal of Solid-State Circuits, vol. SC-24, No. 6, pp. 1517-1522, December 1989. Additionally, a fully static self-calibrated converter and calibration method is discussed in Miller, et al., "A true 16 b self-calibrating BiCMOS DAC", Proceeding of IEEE International Solid-State Circuits Conference--ISSCC '93, pp 58-59, 263, February 1993.
The Miller approach uses an extra D/A converter to correct for the non-ideal characteristics of the main D/A converter, but the range of correction is limited by the sub-D/A converter complexity and the calibration method cannot adjust the full scale gain errors.